I often get asked what type of server a customer should use when landing their virtualised infrastructure, the immediate response is an obvious one, given I work for Intel - an Intel based server ! But beyond this the answer is a little more complex and to some extent depends on the philosophical approach the data centre manager wants to take to architecting their data centre.
There are a number of choices that can be made when using standard Intel based server hardware - ignoring the obvious decision as to the hypervisor vendor - DP ( 2-way ) vs MP ( 4-way ) servers, rack mount vs blade.
Ultimately any server decision is the right one ( so long as its an Intel based solution ) but some of the factors that will influence the decision are :-
How many virtual machines ( VMs ) are you prepared to host onto a single server - MP servers can host substantially more VMs than DP - over 2x more depending on the workload within the VMs - this is down to the better memory capacity and larger number of I/O slots that MP servers typically support compared to DP servers. Against this using a DP server may be a better solution as 2 DP servers may cost less than an MP server, and combined host as many VM's whilst not having as many VMs hosted onto a single server.
Density & Form factor - DP servers typically have a higher density form factor than MP servers - at the expense of less I/O & memory capability. But you need to take into consideration that an MP server can host more VMs than a DP server so within a given rack space use of a lower number of MP servers may well enable hosting of more VMs than using more DP servers.
Blades vs Rack - there is significant momentum building behind the move to bladed servers, mostly driven by the fact that the density achievable using blades is far higher than that possible use rack mount servers. Also the shared resources of a blade solution ( power supplies, cooling, network switches etc ) can lead to cost and power savings in high density configurations. The challenge with hosting a virtualised infrastructure on blade servers however is that blades tend to be limited in the amount of memory and I/O that they can support. The trade off of course is that with the increased density of a blade solution its possible with fewer VMs/Blade but more Blades/rack the overall number of VMs that can be hosted within a given data centre is greater using blades than rack mount servers.
Other factors to take into consideration are that MP servers typically have a higher level of built-in RAS ( reliability, availability & serviceability ) features than DP servers and when hosting multiple VMs on a single server the overall reliability of a servers and its ability to be serviced without shutting down all the hosted VMs becomes very important to the overall efficiency of the Data Centre.
As I said at the beginning - there is no simple answer and a lots depends on the approach you want to take in architecting your solution. Intel's own IT department has done lots of work in this area and have posted many of their results here for others to learn from their experiences.
The only thing that is for certain is that whatever decision is made on form factor the performance of the processors you specify has a direct impact on the number of VMs a server can host - the higher the CPU performance the more VMs that can be hosted and the lower the impact of the hypervisor overhead on the overall system performance, check out the latest virtualisation performance data here and here